Tuesday, June 19, 2007

This is the first in a series of group posts by a few of us bloggers interested in the science of climate change. For our first “mob” post, Tamino at Open Mind, Eli at Rabbet Run and yours truly here at Maribo are all writing about the carbon cycle and atmospheric carbon dioxide.

Much of the discussion on Maribo centers around the science politics of setting a short- and long-term GHG or carbon emissions target in order to stabilize atmospheric concentrations and avoid ‘dangerous’ climate change.

The emissions targets depend on how much - and for how long - the carbon dioxide we emit actually remains in the atmosphere. We need to understand the ability of the planet to take carbon out of the atmosphere, and how that itself is sensitive to climate change. The figure (IPCC WG1, Fig. 7.4) shows the annual fraction of fossil fuel emissions that remained in the atmosphere (black line is a five year mean). I'll come back to this.

The atmosphere is often compared to a bathtub. The emissions of carbon dioxide – the flow into the bathtub – are currently greater than the uptake of carbon – the flow out the drain. So carbon dioxide is accumulating in the atmospheric tub.

Personally, I like to say emissions are currently faster than the planetary uptake. Over geological time, millions of years, carbon is removed from atmosphere by weathering of rock and by burial in marine sediments. Burning fossil fuels releases this ‘fossil’ carbon to the atmosphere; deforestation and biomass burning quickly releases carbon that was stored over decades or centuries in trees. We’ve effectively sped up the flow of carbon into the atmosphere.

The increase in atmospheric CO2 since the Mauna Loa record began in the 1950s is only about half (~55%) of fossil fuel emissions. The rest has been absorbed by the oceans and terrestrial ecosystems.

The ocean ‘sink’ is best understood and easiest to measure. It can be almost entirely explained by the dissolution of CO2 in sea water, the reason the pH of the oceans is declining. Since solubility of CO2 decreases with temperature, much of this uptake has occurred in cold waters of the Southern Ocean. Other potential, but currently negligible on a global scale, ocean sinks include increases in photosynthesis by plankton [and deep-water burial of the ‘fixed carbon’] and changes in ocean circulation.

So we know with good confidence that about 30% of fossil fuel emissions have been absorbed by the oceans and the remainder by terrestrial ecosystems. The remainder must be taken up by terrestrial ecosystems.

The land sink is more challenging to quantify. We know there has been a net uptake of carbon on land. The knowledge of anthropogenic emissions and good estimate of the ocean sink allow us to infer this total land uptake or land sink. So that means carbon uptake by photosynthesis by terrestrial ecosystems is greater than carbon emissions by those ecosystems, from respiration, but also from disturbances like fires and deforestation.

Notice that I did not include deforestation as a CO2 sources above – just fossil fuel emissions. Deforestation is responsible for about 20% of total anthropogenic CO2 emissions; fossil fuels and the like for the other 80%. But since I’m talking about the net exchange of carbon between land and the atmosphere, carbon emissions from deforestation is folded into the equation.

Anyhow, field observations, including forest inventories, satellite observations of terrestrial productivity, data from ‘flux’ towers at specific locations, and modeling point to a few key players:

- Re-growth of forests on abandoned farmland in the Northern Hemisphere has led to a net uptake of carbon (at least until the trees reach maturity)- Higher concentration of atmospheric CO2 can increase rates of photosynthesis and hence carbon uptake (“CO2 fertilization”).- Deposition of nitrogen, emitted by burning of fossil fuels and application of fertilizer, may also be unintentionally ‘fertilizing’ forests

Knowledge of the sinks lets us calculate how anthropogenic CO2 emissions translate into increases in atmospheric concentration. Eli’s post provides a model for doing some simple experiments.

Why does this matter? Our understanding of the modern-day carbon cycle underpins to all that stuff about climate policy that you read, see, hear and smell in the news. Right now, we emit about 8 Gt of C per year, and that translates to, as Tamino points out, an increase of about 2 ppm of CO2/year in atmosphere. But what if climate change alters that way the oceans and the land take up carbon? Then the model has to change.

This is one of the great challenges in climate change science AND climate change policy. To work out what percent reduction is necessary to hit a stabilization level, we need to understand carbon cycle feedbacks: how will climate change alter the fraction of emissions that remain in the atmosphere? Here are three (of many) possible feedback effects:

i) Atmospheric CO2 affect on photosynthesis: Will there be carbon fertilization – higher photosynthesis - or will water stress and nutrient limitation reduce the fertilization affect?ii) Drying in the tropics: Reduced rainfall in the Amazon would reduce carbon uptake and increase carbon release through firesiii) Ocean circulation: A slowing of ocean circulation could limiting productivity in the surface ocean and sinking of carbon (via increasing stratification – topic for another day)

One way to get at these questions is to examine the year-to-year variability in CO2 growth in the atmosphere. What you see in that IPCC figure at the top of the post is that the rate of uptake by the planet varies widely year to year, from less than 20% of emissions, to over 70% of emissions.

There are a few interesting features. The year-to-year variability mostly originates from tropical forest. For example, you can see high airborne fractions or high growth rates during El Nino events (e.g., 1997-1998, 1972-3, 1982-3) due to related droughts (less C uptake) and fires (more C release). That’s not too surprising. It does serve as a warning: future drying in the tropics, due to climate and/or deforestation, could reduce the carbon sink.

In the past, most of the general circulation or climate models used in the IPCC assessments did not included a complete carbon cycle. The atmospheric CO2 concentrations were imposed based on externally generated scenarios. With a complete representation of the carbon cycle, we could instead impose emission, and allow the model to simulate the change in concentrations and uptake by land and oceans.

The latest IPCC assessment includes a comparison of some ‘coupled’ climate-carbon cycle models. All the models predict a decrease in the sink or an increase in the fraction of emissions that remain in the atmosphere. But more on that next time.

14 comments:

good post, only I would add up some more info on the ii) possible feedback effects - a recent study using NOAA AVHHR satellites found no global increase in burned area -from 1980 to 2000. However, it found significant increase in some parts of the world. Still, the recent years experienced redord braking or unusual wild fire activity in many parts of the world (USA, Canada, Russia, Australia...) - see the graphs in my post - http://ac.blog.sme.sk/c/98483/Analyza-lesnych-poziarov-existuje-globalny-trend.html - see the graphs and links inside.

Further, after a wildfire, the ecosystem is "source" of CO2 for several years, due to increased soil respiration...

...unless you consider the recent large jump in the yearly CO2 increment as evidence (from about 1.5ppm/yr before 2002, to about 2ppm thereafter).

This is clearly not due to greater yearly emissions. While these have gone up lately (by about 3% per year), not by anywhere near enough to account for the jump from 1.5 to 2ppm per year.

Of course, this increase might be due to warmer oceans absorbing less CO2, or perhaps a change in wind patterns over oceans that leads to less CO2 evaporation from the ocean water -- but something unusual does seem to be going on. (unusual compared to previous decades of CO2 increases)

True -- a higher fraction of fossil fuel emissions remained in the atmosphere in the first half of this decade (57%) than in the 1990s (50%), though it was 60% in the 1980s (in IPCC WG1, Table 7.1). All I'm saying above is that we can't definitively attribute a relative decrease in the sink to climate-change induced drought in the tropics.

I was sloppy with my statement "recent large jump in the yearly CO2 increment"

I should have made it clear I was talking about an average yearly increase for several years.

Your more detailed analysis shows the same thing that one finds by comparing the average yearly increase for the decade of the 1990's to the average of the yearly increase over the past 5 years or so.

Namely, that over recent years, the yearly increase averages nearly 1/3 higher than the average increase during the nineties, a difference which can't be accounted for by the fairly small (percent wise) increase in emissions in recent times (about 2% -- from a growth rate of 1% pre-2001 to 3% per year therafter)

Depends how you do the math:total land uptake / total emissions (fossil fuels, cement, deforestation) = 28% during the 80s and 90s, according to the IPCC. But this changes every year, as has been alluded to in the previous comments.

I realize there are going to be fluctuations in the amount, but i am looking for a ballpark estimate.

Is it safe to assume that plants are absorbing somewhere around 1/4 the total yearly emissions?

I have read that roughly half the CO2 emitted by humans is reabsorbed, which leaves another half that would need to be absorbed if we were to make the emissions problem "go way".

So, am i correct in assuming that we would have to increase and maintain vegetation at roughly 3X its current level to absorb all the yearly CO2 emissions from human activities? (ie, to absorb the current 25% plus the remaining 50% that is not currently being absorbed)

How reasonable do you think it is to assume that we can do something on that scale -- ie, "plant our way out of" the current emissions problem?

And, given that plants eventually return their CO2 back to the biosphere, is it also safe to assume that one would have to maintain a large scale planting program into perpetuity to offset the inevitable future release?

I've never done back-of-the-envelope calculations like that for carbon sink potential, because though it may appear so at first, the problem is just not that simple, and any answer we derive that way would be a bit reckless. Perhaps someone else is willing to take the bait?

Safe to say modern-day vegetation can't easily become the storeroom for all the world's fossil carbon. For one, once a forest reaches maturity, the exchange of carbon with the atmosphere becomes basically balanced. No net drawdown. So, yep, you'd need to continually be replanting new forests... next question is then, where? Obviously C-fixation by vegetation varies around the planet -- not a lot of big thick tall trees in far north.

I can assure you I wasn't "baiting" you and I realize simple calculations can be deceptive. But they can also sometimes be useful for giving ballpark estimates -- especially when one is trying to determine whether some solution is within the realm of possibility or whether it is essentially "pie in the sky."

Even my rather crude (and admittedly naive) estimates on this are already enough to make me doubt the feasibility of soaking up all the emissions with trees -- though it might make some difference and I think a global planting initiative is well worth pursuing for its other benefits alone.

But I wanted to get the opinion of someone who knows more about this to see if my thoughts were on the right track.

Some of the people who suggest that we can plant our way out of the current emissions problem undoubtedly have ulterior motives (oppose mitigation), but not all. Some are honest, smart and thoughtful people: eg Freeman Dyson.

The report explains how climate change is likely to affect forests as well as how forest conservation and restoration may help mitigate climate change. The report also helps debunk some of the flawed arguments used by logging advocates.